Mobile telephone networks typically include a mobile switching center MSC and a number of registers in the form of databases accessed during the establishment of a telephone call connection or other events in the network. These databases include a home location register HLR and a visitor location register VLR. They store both static (i.e., non-changing) and dynamic (i.e., changing) data related to the subscribers. The static data includes, for example, a list of identifiers of cooperating exchanges interconnected with the mobile switching center, an identification of the services permitted for each subscriber, and parameters for such services. The dynamic data can include, for example, subscriber defined service data such as preferences, mobile station location data, and mobile station activity data.
The HLR contains data needed to establish a telephone connection within the mobile telephone network, so any failure of the HLR will deprive a substantial number of subscribers of service, and correspondingly affect operator revenue. Conventionally, the hardware and software is arranged for high reliability and availability, typically using mirroring techniques and local backup storage. To give some geographical redundancy, each HLR is often coupled to another HLR at a different location, to create a mated pair and the load is split between them, often but not necessarily, in 50—50 proportion. If one has all or the majority of the load, it is referred to as the master HLR, and the other as the slave. The two HLRs are located with sufficient geographical separation to provide resilience to local events such as floods or earthquakes. In the case of a disaster affecting one of them, they are designed to have sufficient capacity for one to handle the entire load. This requires a cutover operation. A GSM/GPRS/UMTS (Global system mobile)/(General Packet Radio Service)/(Universal Mobile Telecommunication System) HLR Mated-Pair Disaster Cutover (also referred to as failover) involves an HLR in a mated-pair arrangement detecting that it's mate HLR has undergone a “disaster” and then “switching over” to provide an active service for the subscribers belonging to the mate HLR (as well as continuing to provide an active service for its set of home subscribers).
Note that the word “disaster” in this context refers to one of the following:
a) one of the HLRs in a mated-pair becomes inaccessible (Total Route Failure) i.e. all network communication is lost to one of the HLRs, and
b) one of the HLRs in a mated-pair goes out of service (Nodal Failure) e.g. caused as a result of a natural phenomena (e.g. Earthquake).
A Mated-Pair Product which uses a Manual cutover mechanism has its limitations in that manual decisions can delay the time to cutover and subsequently may result in a temporary loss of service. A Manual Cutover mechanism has essentially two categories of procedures:
1. Disaster Detection—During Manual Disaster Detection, the operator has to rely on supporting information to assist with the decision to declare a Cutover. This could include potentially some or all of the following:
Manually examine logs generated and alarms raised on the surviving HLR.
Phone calls between Operators (HLR, STP [Signalling Transfer Point])
Increase in SCCP (Signaling connection control part) Maintenance messages, etc.
Manually work out if the HLR is isolated or not.
2. Disaster Cutover—During Manual Disaster Cutover, the operator has to modify some disaster standby configuration data to declare that the HLR is operating Standalone. Also the Operator has to manually run any procedures for aligning operational measurements, etc.
U.S. Pat. No. 5,623,532 discloses a system where two HLRs support each other to provide geographical redundancy, via an SS7 (Signaling System No. 7) telecommunications network without the need for additional links or interface modules between the two mated HLRs. The two HLRs, are connected through the same two Signal Transfer Points (STPs). Each node in a SS7 telecommunications network is supported by dual STPs. In case the first STP or links between the first STP and the destination node fails, the second STP is utilized to provide reliable network operation by passing the messages for the failed HLR to its paired HLR. Determination of failure of an HLR is made manually by an administrator, or by the STPs, not by the paired HLR. The two HLRs do not monitor each other. This patent does not show distinguishing signals communicated between the two HLRs and signals received from other nodes over the network.
U.S. Pat. No. 5,953,662 also shows having two HLRs located anywhere within the SS7 network and supporting each other in real time without requiring additional communications links between the two and without destroying the integrity of the data base. This patent goes further than the '532 patent in that it shows the HLRs sending messages to each other over the SS7 network. One use for such messages is for a first HLR to update the contents of its data base to conform to that of its paired HLR so that it can take over at any time from the paired HLR, and vice versa. The actual transmission is achieved over the same SS7 telecommunications network utilizing the same Signal Transfer Points (STPs).
The HLRs also monitor each other for failure by sending occasional heartbeat messages to each other. A lack of response to a heartbeat is interpreted by a first HLR as indicating a failure of the other HLR. As the lack of response lasts longer, the perceived failure status of the paired HLR is upgraded from temporarily out of contact to inoperable. As before, should the other HLR fail, signals from other entities intended for the other HLR are rerouted by the local STP of the SS7 network to the first HLR for processing.
However there are a number of drawbacks with relying on lack of responses to heartbeats. In particular, lack of response can be caused by congestion, and lead to unwanted cutover.